Unexpected behaviour of Application.OnActivate in Delphi 7

Question

I wrote a D7 application to test the behaviour of Application.OnActivate .
This is the relevant part :

procedure TMainForm.FormCreate (Sender: TObject);
begin
Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.FormCreate - Begin');

  Application.OnActivate := AppActivate;

Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.FormCreate - End');
end;


procedure TMainForm.AppActivate (Sender: TObject);
begin
Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.AppActivate - Begin');

  ShowWidthsHeights (Sender);

Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.AppActivate - End');
end;


procedure TMainForm.ShowWidthsHeights (Sender: TObject);

begin
Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.ShowWidthsHeights -     Begin');

Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.ShowWidthsHeights -     End');
end;

After starting the application , the content of Memo1 is :
1 - MainForm.FormCreate - Begin
2 - MainForm.FormCreate - End
3 - MainForm.AppActivate - Begin
4 - MainForm.ShowWidthsHeights - Begin
5 - MainForm.ShowWidthsHeights - End
6 - MainForm.AppActivate - End

That is correct .

But if I then click on a shortcut in the taskbar , these 4 lines are added to Memo1 :
7 - MainForm.AppActivate - Begin
8 - MainForm.ShowWidthsHeights - Begin
9 - MainForm.ShowWidthsHeights - End
10 - MainForm.AppActivate - End

Why does D7 do that ?
My application hasn't been activated , on the contrary , it has been deactivated !

To test if the Application.OnDeactivate event is also fired , I added this event handler :

procedure TMainForm.AppDeactivate(Sender: TObject);
begin
Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.AppDeactivate - Begin');

Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.AppDeactivate - End');
end;

and added this statement to TMainForm.FormCreate :

Application.OnDeactivate := AppDeactivate;

After starting the application , Memo1 contained the same 6 lines as in the original case , but clicking on a shortcut in the taskbar resulted in 8 extra lines in Memo1 :

7 - MainForm.AppDeactivate - Begin  
8 - MainForm.AppDeactivate - End  
9 - MainForm.AppActivate - Begin  
10 - MainForm.ShowWidthsHeights - Begin  
11 - MainForm.ShowWidthsHeights - End  
12 - MainForm.AppActivate - End  
13 - MainForm.AppDeactivate - Begin  
14 - MainForm.AppDeactivate - End

So , my application gets deactivated , then activated , and then deactivated again !
That's quite confusing !

Delphi is simply reflecting to you the WM_ACTIVATEAPP message that Windows sends to your app. Why Windows does so, I do not know. — David Heffernan
Are you positive that 7 through 10 is logged when you first click on the taskbar button (which would presumably minimize the form), or when you click again to see the memo? You can use OutputDebugString and the 'Event Log' (ctrl+alt+v) instead of logging to the memo to be sure. — Sertac Akyuz
@Sertac Yeah, I checked this already. It is exactly as described in the Q. — David Heffernan
Well, I can't reproduce it. I get a nice app-deactivate and that's it when I click on the taskbar. I suspect the OP is doing something else (double-clicking). — Marck

R-D R-D · Accepted Answer · 2012-06-27T12:11:04

As the comment of David already stated, it is reflecting the receiving of the WM_ACTIVATE message. http://msdn.microsoft.com/en-us/library/windows/desktop/ms632614(v=vs.85).aspx gives the answer:

Sent when a window belonging to a different application than the active window is about to be activated. The message is sent to the application whose window is being activated and to the application whose window is being deactivated.

and the parameters of the message explain further:

wParam:

Indicates whether the window is being activated or deactivated. This parameter is TRUE if the window is being activated; it is FALSE if the window is being deactivated.

Unexpected behaviour of Application.OnActivate in Delphi 7

1 Answers